Passivating beryllium



Jan. 31, 1967 5. J. MORANA 3,301,718

PASSIVATING BERYLLIUM Filed March 22, 1965 me. FIG.IB

FIG.2A HG. 2B

FIG .3A

FIG. 4B

Simou MORAN) WJ Wm United States Patent 3,301,718 PASSIVATING BERYLLIUM Simon J. Morana, Hazleton, Pa., assignor to The Beryllium Corporation, Reading, Pa., a corporation of Pennsylvania Filed Mar. 22, 1965, Ser. No. 442,240 11 Claims. (Cl. 148-6.16)

This application is a continuation in part of my copendin'g application Serial No. 260,857, filed February 25, 1963, now abandoned.

This invention relates to a process of passivating beryllium metal and the product thereof. More specifically it deals with a process of surface treating beryllium metal in all forms, either powder or shapes, which renders the same resistant to chemical oxidation or corrosion.

It is therefore an object of the. present invention to provide a process for treating beryllium metal to render it more inert and less reactive.

An object of the instant invention is to provide a beryllium metal product either in powder form or shapes which is resistant to oxidation.

A further objectof the instant invention is to provide coating solution for beryllium metal which will render the metal corrosion resistant.

A still further object of the instant invention is to provide a process whereby beryllium metal in any form is subjected to a chromium containing coating which will render the metal relatively inert and resistant to oxidation or corrosion.

Other objects and advantages of the instant invention will become apparent from the following detailed description and examples.

FIGS. 1a and 1b are photographs of both sides of beryllium discs showing the hydrolysis product on the surfaces;

FIGS. 2a and 2b are photographs of both sides of the beryllium disc shown in FIGS. 1a and lb after removal of the hydrolysis pro-duct by acid and showing the deep pitting of the metal;

FIGS. 3a and 3b are photographs of both sides of beryllium discs which have been treated in accordance with the present invention and subjected to corrosive environment; and

FIGS. 4a and 4b are photographs of both sides of the beryllium discs shown in FIGS. 3a and 3b after treatment with acid to remove any hydrolysis product.

Beryllium is a strong, light weight meta-l receiving much attention as a material of construction in specialized fields. An impediment to its use is the fact that the metal tends to corrode or become pitted when subject to moisture for long periods of time. This corrosion takes place on the exposed surface of the metal and when submerged in Water it will occur over the entire surface with deep pitting occurring in areas anodic to the bulk of the metal. The corrosion takes the form of snow white hydrolysis product on the surface of the exposed metal and is usually reported as a weight gain or if the same is removed, as a weight loss. This hydrolysis product is beryllium hydroxide as represented by the following chemical equation.

' 'It has been found that by treating beryllium metal with a chromium containing solution, a coating is formed on the metal which will prevent oxidation or corrosion of the pick up of from .2 to .9 percent.

dilution of 5 to 1 passivity of the beryllium metal dehce metal. The passivation of the beryllium metal allows the metal to be used in fields where corrosion resistance is desirable and also increases the shelf life of the metal during storage.

The chromium containing solution used for carrying out the surface treatment of the metal, comprises equal parts by volume of a saturated aqueous solution of potassium dichromate and concentrated phosphoric acid. This solution may be diluted to various concentrations by the addition of water. It is also possible to use potassium dichromate in various dilutions of water or even chromic anhydride in various dilutions of water. All of these solutions will form a chromium-containing complex on the beryllium metal producing the surface passivity.

The activity of the reaction resulting from mixing the chromium containing solutions with water decreases as the degree of dilutions of the stock of the chromium containing solution increases. As an example use may be made of a mixture of equal parts by volume of concentrated phosphoric acid and a saturated aqueous solu tion of potassium dichromate with equal parts by volume of water, which gives an active reaction with beryllium powder, and dilutions of as much as 50 parts water and 1 part of the phosphoric acid dichromate solution, while exhibiting a very mild reaction withberyllium powder give to the powder very excellent surface passivity.

Using a dilution of 20 parts water to 1 part by volume of the phosphoric acid chromate solution results in a pick up of .03 to .5 percent chromium and .01 to .10 percent phosphorous in the surface treated powder, depending on the particle size of the powder.

Another stock solution employed for effecting the desired surface treatment consists of a saturated aqueous solution of potassium dichromate which when diluted with not more than 5 parts by volume of water results in excellent surface treatment passivity.--- Using a 1 to 1 dilution of this solution with water results in a chromium In this case above a creases unless phosphoric acid is added.

As mentioned above another surface treatment process,

involves the use of a stock solution consisting of phosphoric acid solution of ohromic anhydride, CrO equivalent to the chromium content of the saturated dichromate solution. A 1 to 20 dilution of this solution with water may be used and beryllium powder mixed therein has an excellent surface passivity.

While it is clearly not understood what exact mechanism is involved in connection with the surface treatment which results in the passivity of the metal particles, X-ray diffraction studies indicate that the surface treated powders contain less beryllium oxide than the untreated berryllium powders.

While the chromium coating received by the powdered particlesmay be of varying thickness, it has been calculated that a monomolecular coating of chromium on the surface of a 4-micron particle size berryllium sphere would result in 0.101 percent pickup.

As a more specific illustration of the process of the invention, the following example is given:

5 gallons of surface coating solution consisting of 1 pint of reagent grade phosphoric acid, 1 pint of saturated solution of potassium dichromate and 4% gallons of water, cooled to room temperature was placed in a stainless steel vessel fitted wit-h an agitator. 6 pounds of nominal 10 micron beryllium subsieve powder was gradually added over a period of 10 minutes, with agitation, and the agitation was continued an additional 20 minutes.

test and did not allow the entrance of further air as was the case in A and B, which were in beakers covered with a loose fitting cover glass.

The rate of weight gain described in Table I can be The slurry was filtered and the filter cake was washed 5 equilibrated to the actual weight loss of beryllium metal with 5 gallons of water and refiltere-d. by taking into account the chemical equation above which The contained water was removed by washing with describes the mechanism for hydrolysis of the beryllium acetone followed by a hydrocarbon solvent, such as whereby 9 grams of Be react upon 43 grams of Be(O-H) Stoddard solvent, and the resulting filter cake is dried The majority of the erosion of beryllium metal under under vacuum to produce the surface treated powder. 10 these conditions occurs in sporadic deep pitting of the The solution of the foregoing example gives a solution metal, thus weakening its structural qualities. of the acid dichromate mixture of about 1 to 19. The reaction with the beryllium powder is relatively mild Table II and gives a surface Coated Powder having excellent [Corrosion rate of Be metal submerged in water at 100 0.] passivity.

An accelerated test has been devised whereby test Sample Surface Treatment Corrosion rate, beryllium metal discs 2 inches in diameter by inch mgJSq-m/Year thick were submerged in water in an oven maintained at 9 9 month periods- Untreated beryllium 3:1:11:31:::::::::::::I::::: Eight- 311355555511: :93 metal d1scs invariably become completely coated with a 20 white covering with additional white beryllium hydroxide suspended in the water under these conditions. The The solution used for coating the beryllium discs comcoatings of the instant invention are of a thickness on prised of 1 part by volume saturated potassium dichrothe order of magnitude of to 50 Angstrom units and mate solution, 1 part by volume 85% phosphoric acid, do not change the dimensional tolerances nor the metallic 25 and 2 parts by volume of water. The parts were then waappearance of the coated beryllium metal. The passivater washed and dried. It is understood, however, that tion coating consists of a chromium complex compound, other chromium compounds and other dilutions can be which is believed to be a complex beryllium-chromiumsimilarly used. phosphorous compound, which is app-lied by simply In another accelerated corrosion test involving a cyclidipping the beryllium metal part in an aqueous solution cal temperature-humidity environment sixteen test discs composed of potassium dichromate and phosphoric acid, prepared from varying beryllium compositions were followed by a water rinse. treated with the chrome phosphorous coating and tested. To illustrate the passivation effect of the coatings, Table The corrosion test is considered completed when any I describes the results of submerging beryllium test discs evidence of corrosion or pitting is observed. All sixteen 2 inches in diameter'by inch thick in distilled water passivated test discs showed no evidence of corrosion in glass beakers placed in an oven heated at 100 C. after 5,000 hours, whereas some other test discs prepared Bare beryllium test discs or untreated beryllium test by standard electrolytic anodization procedures failed discs were degreased with solvent to remove surface films. after 700 hours. The other discs were surface treated as described. These chrome phosphorous coatings have also been Table 1 Rate of Days Weight Weight Weight Weight Sample Surface Treatment; in at Start, at End, Gain, Gains,

No. Water gms. gms. gins. lug/sq. in./

year

8 3. 5989 3. 5989 None None 30 3.5930 3.5991 .0002 .4 7 3. 5139 3. 5139 None None 7 3.5760 3.5876 .0020 21.4

Sample A2Completely covered with a heavy white found to be protective at elevated temperatures, as evicorrosion film on both sides of disc, white solids in denced by the results of the following testing: suspension in water (see FIGS. 1a and 1b and FIGS. A beryllium test disc (sample I) was dipped once in 2a and 2b). 5 the potassium dichromate-phosphoric acid solution and Sample B2Disc remained bright and shiny except for air dried. Another test disc (sample II) was dipped in the two bluish-white spots on one side. Water remained same solution, air dried, and was given a second dipfolclear (see FIGS. 3a and 3b and FIGS. 4a and 4b). lowed by air drying. A bare beryllium test disc (sample Sample CDisc remained bright and shiny, water re- III) was placed together with the two treated discs into a mained clear. laboratory mufifle furnace preheated to 1400 F. and Sample DDisc had a satiny thin white coating on both maintained at that temperature for 18 hours before being sides interpersed with several white spots, water was removed fr m th f rnace. Th muflle furnace w s s bhazy. jected to atmospheric air by mean-s of periodic opening and closing the door and through thermocouple holes in The slightly lower rate of weight gain in sample D is the mufile furnace. The test results are summarized in Table III.

. Table III [Elevated temperature corrosion resistance] 6 Table IV [Effect of passivation temperature] Sample No. Gms. at Gms. at Weight Percent Start .End Gain, gins. Gain As stated above it has been found that dilution of the stock solution of phosphoric acid and dichromate decreases the passivity given to the metal as dilution increases. Some tests showing various dilutions of the stock solution with'various volumes of water are shown below.

Beryllium test discs 2 inches diameter by inch thick were dipped for 30 minutes in a solution comprised of 1 part by volume saturated potassium diehromate solution, 1 part by volume 85% phosphoric acid, and varying dilution volumes of water as indicated in the data below. The treated discs were water washed, air dried and placed into individual 600 ml. Pyrex beakers filled with distilled water and covered with loose fitting cover glasses to minimize'evaporation. The beakers were placed into an oven controlled at 100 C. for 30 days, periodically replenishing any evaporated water with fresh distilled water as needed in order to maintain the discs submerged throughout the test. Following are the test data obtained. The volumes of water added indicates the volume of dilution water added to the saturated potassium dichromate and phosphoric acid dip solution for each test.

Weight Weight Weight at start, at end, gain, gins. gins. gms.

Volumes of water added:

The above data indicates that although the best passivation protection is obtained by using only 2 volumes of water per 1 volume of saturated potassium dichromate and ,1 volume'of 85% phosphoric acid, some protection is also afforded by diluting with as much as 30 volumes Of.'Wate1. With regard to the passivation of beryllium powder it has been found that variation in temperature, magnesium content and particle size will give different results for the powder., Thisfactor is due to the large surface area present in'the powder form. The variation in temperature is 'ineantvariation in the temperature at which the coating reaction takes place. It has been found that certain temperatures produce a greater amount of passivity in the beryllium metal powders than others. This tem perature is effected by the magnesium content of metal and the particle size of the metal. The temperature at which the greatest amount of passivity is achieved varies inversely with the magnesium content of the metal and directly with the particle size. It was determined that variables such as particle size of the beryllium powder or the magnesium content of the powder to be passivated can vary the weight gain or loss in the corrosion test by as much as 18%.

With regard to shapes, rather than powder form of beryllium metal, the surface area is so much less in the shape, that the effects of these variables are negligible.

Table IV summarizes experimental data obtained by passivating at different'temperatures beryllium powders with different average particle sizes and powders containing low (less than .1%) and high (1%) magnesium. Average particle size is defined as that point on a curve plotting particle size vs. cumulative weight percent which represents 50% of the cumulative weight. Particle size determination is made by Coulter counter method.

Mg. Treat Weight Average Size, Content, Temp, Gain Percent Percent microns Percent C. (or loss), Gr.

Percent The above data clearly demonstrates that the average particle size of the powder to be treated as well as the magnesium content, must be taken into account to select the proper passivation temperature.

Thus, the reaction temperature for a low magnesium 10-micron average beryllium powder should be about 30 C. whereas the reaction temperature for a similar size powder with a high magnesium content should be about 10 C. Similarly, whereas a temperature of about 70 C. is required to fully passivate a 23-micron average particle size powder, that same reaction temperature for a 10-micron particle size powder, would result in a passi: vated powder with an assay of only 89%. With a 7- micron particle size powder that same reaction temperature would result in a beryllium powder containing an assay of about 80%.

It is apparent from the foregoing that there is provided a new and novel process for passivating beryllium metal, shapes or powder by treating said metal with an aqueous chromium containing solution which coats the metal with a chromium complex which causes the metal to become relatively inert and resistant to oxidation or corrosion. The ohromium'containing solution may be of various chromium compounds with or without phosphoric acid and water. The amount of dilution with water can vary greatly as set forth above. The method of applying the coating as set forth above is by dipping, however, any other method which would be effective in coating the metal may be used. When dealing with powders it is more effective to determine the magnesium content and the size of the powder so that the proper temperatures can be used during the coating action for best results.

The beryllium met-a1 powders which can be passivated by the above recited process are of a weighted average particle size of from about 1 micron to about microns. As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, and since the scope of the invention is defined by the appended claims, all changes that fall in the metes and bounds of the claims .or that form their functional as well as conjointly cooperative equivalents are therefore intended to be embraced by those claims. p

I claim:

1. The process of imparting surface passivity to metal consisting essentially of beryllium which comprises contacting the beryllium metal with an aqueous mixture prepared from a saturated solution of hexavalent chromium ions diluted with water in an amount of up to 50 times its volume and sufiicient phosphoric acid to increase the passivity of the coating, thereby effecting deposition of a passivating coating on the metal.

2. The process of claim 1, wherein the chromium containing solution is an aqueous mixture of a member of the group consisting of potassium dichromate and chromic anhydride.

3. The process of claim 1, wherein the beryllium metal is essentially beryllium metal powder of a weighted average particle size of from about 1 to about 100 microns.

4. The process of claim 3, wherein the temperature varies from about 5 degrees to about 80 degrees C.

5. The process of imparting a surface passivity to beryllium metal powder particles wherein the beryllium metal particles are essentially beryllium metal particles of a weighted average particle size of from about 1 to about 100 microns which comprises mixing beryllium metal powder into an aqueous mixture prepared from a saturated solution of hexavalent chromium ions diluted with water in an amount of up to 50 times its volume and sufiicient phosphoric acid to increase the passivity of the coating, and thereby eiiecting deposition of a passivating coating on the powder particles.

6. The process of treating beryllium metal powder wherein the beryllium metal powder is essentially beryllium of a weighted average particle size of from about 1 to about 100 microns which comprises introducing the metal powder into an aqueous solution prepared from a saturated solution of potassium dichromate and concentrated phosphoric acid, retaining the powder in the solution for a period of approximately 30 minutes, filtering to obtain a beryllium powder cake, and then dewatering the powder cake to obtain beryllium metal powder in a passive state.

7. The process of treating beryllium metal powder wherein the beryllium metal powder is essentially beryllium of a weighted average particle size of from about 1 to about 100 microns which comprises forming a stock solution consisting of equal parts of a saturated solution of potassium dichromate and concentrated phosphoric acid, preparing a dilution of the stock solution of from 1 to 1 to 1 to 50 parts Water, introducing the beryllium metal powder into the solution with stirring and retaining the powder therein for a least minutes, then filtering to recover the powder, washing the powder and finally dewatering and drying the powder to obtain a surface passive beryllium metal powder.

8. The process of treating beryllium metal powder a beryllium metal powder having from about .03 to about 0.5 percent of chromium and from about .01 to about .10 percent phosphorous on the particles by filtering the mixture, then washing the powder and finally dewatering and drying the powder.

9. A method of preventing corrosion of metal consisting essentially of beryllium by oxidation comprising coating the metal with an aqueous mixture prepared from a saturated solution of hexavalent chromium ions diluted with water in an amount of up to 50 times its volume and sufii'cient phosphoric acid to increase the passivity of the coating thereby forming a passivating coating on the surface of the metal and rendering the beryllium metal resistant to oxidation.

10. Metal consisting essentially of beryllium having a surface passivating coating thereon prepared by contacting the metal with an aqueous mixture prepared from a saturated solution of hexavalent chromium ions diluted with water in an amount of up to 50 times its volume and sufficient phosphoric acid to increase the passivity of the coating thereby rendering the beryllium metal resistant to chemical oxidation or corrosion.

11. Beryllium metal according to claim 10, wherein the metal is a powder having an average weighted particle size of from about 1 micron to about microns.

References Cited by the Examiner UNITED STATES PATENTS 2,445,155 7/1948 Saukaitis 148-62 2,516,685 7/1950 Douty et al. 148--6.16 X 2,656,322 10/1953 Eberle 148--6.2 X 2,729,601 1/1956 Beach et al 204140.5 X 2,882,189 4/1959 Russell et al 148--6.16 2,970,898 2/1961 Fox 14976 3,002,830 10/1961 Barr 149-19 3,070,469 12/1962 Jenkins 1495 3,133,841 5/1964 Kuehl 149-5 3,140,204 7/ 1964 Tokunaga 148-6.2 3,183,125 5/1965 Bjork 1486.2

OTHER REFERENCES White et al.: The Metal Beryllium, The American Society for Metals, Cleveland, Ohio, 1955, pp. 545-547.

ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner. 

1. THE PROCESS OF IMPARTING SURFACE PASSIVITY TO METAL CONSISTING ESSENTIALLY OF BERYLLIUM WHICH COMPRISES CONTACTING THE BERYLLIUM METAL WITH AN AQUEOUS MIXTURE PREPARED FROM A SATURATED SOLUTION OF HEXZAVALENT CHROMIUM IONS DILUTED WITH WATER IN AN AMOUNT OF UP TO 50 TIMES ITS VOLUME AND SUFFICIENT PHOSPHORIC ACID TO INCREASE THE PASSIVITY OF THE COATING, THEREBY EFFECTING DEPOSITION OF A PASSIVATING COATING ON THE METAL. 